Fusion with activated mouse oocytes modulates the transcriptional activity of introduced somatic cell nuclei

We have analyzed the transcriptional activity of somatic cell nuclei fused with artificially activated mouse oocytes. Two types of somatic cells have been used: transcriptionally silent thymocytes, obtained from the thymus of newborn mice, and transcriptionally active murine erythroleukemia cells (MEL) from in vitro culture. Cells were fused with activated oocytes, either less than 1 h, or 3 h post-ethanol-treatment. When the fusion occurred early after activation (1 h or less), the transferred somatic cell nuclei reacted by nuclear envelope breakdown (NEBD), which exposed their chromatin to the cytoplasmic environment of the oocytes, and the reconstituted nuclei underwent significant ultrastructural remodeling. No transcriptional activity was ever detected in these reconstituted nuclei during the subsequent 3-4 h of culture of the resulting hybrid cells. In the case of MEL, this means that transcriptional activity ceased as soon as they entered the cytoplasm. In contrast, somatic nuclei which entered the cytoplasm of activated oocytes 3 h postactivation did not undergo NEBD and their remodeling was less pronounced. In contrast to the first group, these nuclei were transcriptionally active during the following 3-4 h of culture. In both cases, the female pronucleus remained transcriptionally silent.